Regarding lithium ion batteries in the vehicle and the like, a property (so-called a shut down function) to automatically and safely terminate discharge and charge of the battery when an accident such as malfunction occurs, has been required. In the battery, the separator is provided with such property. Usually, the separator is designed so that when the temperature is approximately 110 to 140° C., the separator fuses to block micropores, thereby blocking Li ions, leading to termination of the electrode reaction under over-heated circumstances. However, there are cases where the shut down by the separator is incomplete and thus the temperature increases to above the melting point of the separator, and cases where the temperature increase in the external surroundings result in the meltdown of the separator. Such cases would result in an internal short-circuit. Then, the shut down function of the separator can no longer be counted on, and the battery would be in the state of thermal runaway. In order to deal with such circumstances, a technique to form a positive temperature coefficient resistor on the current collector has been suggested.
For example, as a technique to form the positive temperature coefficient resistor on the current collector, Patent Literature 1 discloses a technique in which the surface of the current collector is coated with a conductive layer comprising a crystalline thermoplastic resin having a function as the positive temperature coefficient resistor, a conductive agent, and a binding agent. Here, the function as the positive temperature coefficient resistor is the function where the resistance value increases along with the increase in temperature. According to such technique, when the temperature inside the battery reaches the melting point of the crystalline thermoplastic resin by the heat generated due to the overcharge of the battery, the resistance of the conductive layer rises sharply, thereby cutting off the current which is flowing through the current collectors to realize the shut down function.
In addition, in Patent Literature 2, a technique to embed conductive fillers in PTC powders having a resistance value which increase along with the increase in temperature, and the arranging the PTC powders in the positive electrode material layer, is disclosed.
In Patent Literature 3, a technique to arrange thermally expandable microcapsules in between electrodes is disclosed. Here, when the microcapsules expand, the conductive path is cut down, thereby achieving a function as a fuse.
In Patent Literature 4, a technique to adhere microcapsules including a substance which inhibits battery reaction onto the surface of the electrode is disclosed. When the microcapsules melt by the increase in temperature, the substance included would be released, thereby achieving a function as a fuse.